1. Field of the Invention
The present invention relates to an audio signal processing apparatus that converts a digital audio signal to be inputted thereto into an analog audio signal. More particularly, the present invention relates to a multi-channel audio signal processing apparatus that simultaneously processes a plurality of audio signals.
2. Description of the Related Art
In a digital audio device, an audio signal processing apparatus that converts a digital audio signal to be inputted thereto into an analog audio signal is used. As a format of a digital audio signal to be inputted, an I2S format, for example, is known.
FIG. 6 is a diagram showing signal waveforms of digital audio signals transmitted in the I2S format (hereinafter, referred to as the “I2S signals”). The I2S signals include a DATA signal in which L-channel audio data and R-channel audio data are alternately arranged on a word-data basis; a word clock signal (hereinafter, referred to as the “LRCK signal”) for identifying word data of the DATA signal; and a bit clock signal (hereinafter, referred to as the “BCLK signal”) for identifying each of bit data constituting word data.
The DATA signal is serial data (DL1/DR1, DL2/DR2, . . . DLm/DRm) obtained by making a pair L-channel data DLi (corresponding to n-bit data and one word data) and R-channel data DRi (corresponding to n-bit data and one word data) which are at the same sampling location i and arranging pairs in order of sampling. The LRCK signal is a clock whose one cycle corresponds to one-word data DLi/DRi of the DATA signal. In FIG. 6, a low-level (hereinafter, referred to as the “L-level”) period of the LRCK signal is synchronized with L-channel word data DLi of the DATA signal and a high-level (hereinafter, referred to as the “H-level”) period of the LRCK signal is synchronized with R-channel word data DRi of the DATA signal. The BCLK signal is a clock that is synchronized with bit data of the DATA signal.
The DATA signal is divided into L-channel word data DLi and R-channel word data DRi using the LRCK signal, and is converted into an analog signal on a bit-data basis using the BCLK signal. By this, the I2S signals are converted into an L-channel-data-based analog audio signal and an R-channel-data-based analog audio signal.
In the case of multi-channel digital audio signals, a plurality of DATA signals may be used. Some digital audio devices, as will be described below, not only convert digital audio signals of various channels included in a DATA signal into analog audio signals but also generate and output an analog audio signal in which a combination of two audio signals different from a combination of the two audio signals included in the DATA signal is mixed.
FIG. 6 shows I2S signals for a 5.1ch surround speaker system. Note that, in the 5.1ch surround speaker system, six speakers are placed around a listener (see FIG. 5A), including an L-channel front speaker (a speaker on the forward left side of the listener; an FL speaker), an R-channel front speaker (a speaker on the forward right side of the listener; an FR speaker), an L-channel rear speaker (a speaker on the rear left side of the listener; an SL speaker), an R-channel rear speaker (a speaker on the rear right side of the listener; an SR speaker), a center speaker (a speaker at the forward center of the listener; a C speaker), and a subwoofer speaker (a speaker dedicated to bass sounds; an SW speaker); therefore, DATA signals of I2S signals for 5.1ch surround include six types of digital audio signals for their corresponding speakers. In FIG. 6, however, for convenience of description, digital audio signals for the C speaker and the SW speaker, which are included in the DATA signals, are not shown. In the following description, in some instances, the “channel” is denoted as “ch”.
DATA signals of I2S signals for 5.1ch surround include an FL_FR signal in which audio data (FL data) which is converted into audio to be outputted from the FL speaker and audio data (FR data) which is converted into audio to be outputted from the FR speaker are combined; and an SL_SR signal in which audio data (SL data) which is converted into audio to be outputted from the SL speaker and audio data (SR data) which is converted into audio to be outputted from the SR speaker are combined.
The FL_FR signal included in the DATA signals of the I2S signals shown in FIG. 6 is divided into FL data and FR data and the SL_SR signal is divided into SL data and SR data. The FL data, the FR data, the SL data, and the SR data are respectively converted into an fl signal, an fr signal, an sl signal, and an sr signal which are analog audio signals. The converted fl signal, fr signal, sl signal, and sr signal are respectively outputted to the FL speaker, the FR speaker, the SL speaker, and the SR speaker. Note that a digital audio signal for the C speaker is converted into a c signal which is an analog audio signal and outputted to the C speaker and a digital audio signal for the SW speaker is converted into an sw signal which is an analog audio signal and outputted to the SW speaker.
A digital audio device (audio amplifier) which is applied to a 5.1ch surround speaker system is, as described above, provided with output terminals for six analog audio signals (an fl signal, an fr signal, an sl signal, an sr signal, a c signal, and an sw signal) for six speakers; however, when the user does not have a 5.1ch surround speaker system, those output terminals that do not have their corresponding speakers are not connected and thus analog audio signals for those output terminals are not used.
For example, when a speaker system owned by the user is one that includes an FL speaker, an FR speaker, and a C speaker, in this speaker system, an sl signal, an sr signal, and an sw signal cannot be used. To overcome such inconvenience, some audio amplifiers which are applicable to a 5.1ch surround speaker system are configured to output a signal (fl_sl signal) in which an fl signal and an sl signal are mixed, from an output terminal for the fl signal and output a signal (fr_sr signal) in which an fr signal and an sr signal are mixed, from an output terminal for the fr signal. When such audio amplifiers are combined with a speaker system including an FL speaker, an FR speaker, and a C speaker, by causing the FL speaker and the FR speaker to respectively output an fl_sl signal and an fr_sr signal as audio signals, both an sl signal and an sr signal can be effectively used.
FIG. 7 is a diagram for describing an audio signal processing apparatus capable of generating analog audio signals, i.e., an fl signal, an fr signal, an sl signal, an sr signal, an fl_sl signal, and an fr_sr signal, from digital audio signals, i.e., an FL_FR signal and an SL_SR signal, and outputting two different types of combinations by switching therebetween. An audio signal processing apparatus A100 includes DA converter circuits 310 and 320, differential circuits 410 and 420, switching circuits 510 and 520, and output terminals 610a, 610b, 620a, and 620b. 
The DA converter circuits 310 and 320 accept as input DATA signals, an LRCK signal, and a BCLK signal and output converted analog audio signals. The DA converter circuit 310 includes a one-bit DAC 310a and low-pass filters 310b and 310c. The one-bit DAC 310a divides an FL_FR signal into FL data and FR data, performs DA conversion on the FL data and the FR data, and outputs an fl signal and an fr signal. The low-pass filters 310b and 310c respectively remove high-frequency components from the fl signal and the fr signal which are inputted thereto from the one-bit DAC 310a. The DA converter circuit 320 includes a one-bit DAC 320a and low-pass filters 320b and 320c. The DA converter circuit 320 converts an SL_SR signal into an sl signal and an sr signal, removes high-frequency components from the sl signal and the sr signal, and outputs the resulting signals.
The differential circuit 410 combines the fl signal inputted thereto from the DA converter circuit 310 with the sl signal inputted thereto from the DA converter circuit 320 and outputs an fl_sl signal. The differential circuit 420 combines the fr signal inputted thereto from the DA converter circuit 310 with the sr signal inputted thereto from the DA converter circuit 320 and outputs an fr_sr signal.
The switching circuit 510 switches between the fl signal outputted from the DA converter circuit 310 and the fl_sl signal outputted from the differential circuit 410 and causes the output terminal 610a to output either signal. The switching circuit 520 switches between the fr signal outputted from the DA converter circuit 310 and the fr_sr signal outputted from the differential circuit 420 and causes the output terminal 620a to output either signal. The switching circuits 510 and 520 are switched in a coordinated manner and the four output terminals 610a, 610b, 620a, and 620b output one of a combination of (an fl signal, an sl signal, an fr signal, and an sr signal) and a combination of (an fl_sl signal, an sl signal, an fr_sr signal, and an sr signal).
The audio signal processing apparatus A100 is implemented by mounting the DA converter circuits 310 and 320, the differential circuits 410 and 420, and the switching circuits 510 and 520 on a substrate and providing pattern wiring between the circuits. At this time, in order that the fl signal outputted from the low-pass filter 310b of the DA converter circuit 310 is inputted to the differential circuit 410, an output of the low-pass filter 310b and an input of the differential circuit 410 need to be pattern-wired, and in order that the fr signal outputted from the low-pass filter 310c is inputted to the differential circuit 420, an output of the low-pass filter 310c and an input of the differential circuit 420 need to be pattern-wired. In addition, an output of the low-pass filter 320b and an input of the differential circuit 410 need to be pattern-wired and an output of the low-pass filter 320c and an input of the differential circuit 420 need to be pattern-wired.
However, as shown in FIG. 7, since the pattern wiring for the fr signal between the DA converter circuit 310 and the differential circuit 420 intersects with the pattern wiring for the sl signal between the DA converter circuit 320 and the differential circuit 410, these pattern wirings cannot be wired on the same plane of the substrate by patterning. Hence, measures need to be taken such as detouring one of the pattern wirings by a jumper wire or detouring one of the pattern wirings by a through-hole and a pattern wiring formed on the back side. As a result, the wirings on the substrate become complicated and a detoured pattern wiring has a longer line length than the other pattern wiring and thus noise is more likely to be superimposed on an analog audio signal flowing through the detoured pattern wiring, causing a problem that sound quality is adversely affected thereby.